A conventional television receiver, such as a receiver in accordance with the NTSC broadcast standard adopted in the United States and elsewhere, has a 4.times.3 aspect ratio (the ratio of the width to the height of a displayed image). Recently, there has been interest in using higher aspect ratios for television systems, such as 2.times.1, 5.times.3 or 16.times.9, since such higher aspect ratios more nearly approximate or equal the aspect ratio of the human eye than does the 4.times.3 aspect ratio of a standard television receiver display.
It is desirable for widescreen television systems to be compatible with standard television receivers to facilitate the widespread adoption of widescreen systems. One known compatible widescreen television system is described by M. A. Isnardi et al. in an article "Encoding for Compatibility and Recoverability in the ACTV System", published in IEEE Transactions on Broadcasting, Vol. BC-33, December 1987, and in U.S. Pat. No. 4,855,811 of M. A. Isnardi. This known system develops a single channel compatible widescreen television signal which produces a standard 4.times.3 aspect ratio display when received by a conventional television receiver, and which produces a widescreen 5.times.3 aspect ratio display when received by a widescreen television receiver.
The widescreen television system proposed by Isnardi was described in the context of a system wherein the compatible 4.times.3 main image area is horizontally centered on the widescreen image. However, when the main image action is off-center, it may be desirable for the compatible 4.times.3 NTSC display to follow the main action. That is, it may be desirable to steer the standard 4.times.3 display area so that it follows the main image action. Such a procedure is well known and is commonly referred to as "panning" or "pan and scan", wherein a panning window follows the main image action. To implement panning, an operator such as a studio technician monitors the widescreen television signal image before transmission, and manually controls the panning window so that it follows the main image action. Such control simultaneously produces a panning control signal representative of the position of the panning window relative to a nominal center position. The control signal can be transmitted with the television signal, e.g., in the vertical blanking interval.